Pyrogenically produced metal oxide particles, such as fumed metal oxides, are typically prepared by passing a metal oxide precursor compound, such as a metal tetrachloride compound, through a flame. The metal oxide precursor compound is converted into small, spherical molten particles of metal oxide referred to as primary particles. During the process, the primary particles collide with one another, such that the primary particles become fused together into larger, three-dimensional, chain-like aggregate particles. The aggregate particles also can become entangled with one another to form even larger agglomerated particles. Agglomerates, however, are loosely associated structures that can break down into individual aggregate particles under normal dispersion conditions. The primary particles that form the aggregates, by contrast, are held together by strong forces and will not usually break apart upon dispersing the aggregate particles under normal dispersion conditions.
Since the aggregate particle is comprised of fused primary particles, the aggregate particle diameter of metal oxide particles prepared by conventional pyrogenic methods is a function of the primary particle diameter. The primary particle diameter, in turn, determines the surface area of the metal oxide particle. Thus, the aggregate particle diameter of these types of metal oxide particles is related to the surface area of the particles. This detracts from the usefulness of such dispersions when, for example, a smaller aggregate particle diameter is needed than is available for a given surface area.
A need exists for a method of preparing dispersions of metal oxide particles having desired aggregate particle diameters. The invention described herein provides such a method. These and other advantages of the invention will be apparent from the description of the invention provided herein.